Ferroelectric ceramics exhibit extremely high dielectric constant values at low electric fields. However, capacitors which utilize ferroelectric ceramic dielectrics exhibit a significant decrease in capacitance with increasing levels of applied electric field. More specifically, conventional ferroelectric capacitors store a larger portion of their total charge at low voltage, as compared with linear dielectric capacitors which maintain a constant capacitance, independent of applied voltage. Accordingly, such ferroelectrics have not been favored for high voltage applications.
Recently, energy storage capacitors utilizing antiferroelectric ceramic layers have been proposed for use in cardiac defibrillators. Such an application is disclosed in U.S. Pat. No. 5,545,184 to Dougherty, entitled "Cardiac Defibrillator with High Energy Storage Antiferroelectric Capacitor". Cardiac defibrillator applications require an applied voltage of approximately 800 volts, with an energy storage capacity of up to 40 Joules. A class of non-linear, anti-polar ceramic materials, termed antiferroelectrics, exhibit increasing dielectric constants as an applied electric field is increased. At a certain applied electric field, an antiferroelectric undergoes a phase transition to a ferroelectric phase (exhibiting a polar characteristic) and then shows a decrease in dielectric constant at even higher potentials.
The aforesaid Dougherty patent (U.S. Pat. No. 5,545,184) illustrates that capacitors with anti-ferroelectric dielectrics match the conditions required for defibrillation applications. In U.S. patent application Ser. No. 08/688,883 now U.S. Pat. No. 5,728,138, entitled "Cardiac Defibrillator with Multi-phase Ferroelectric/Antiferroelectric Capacitor", Dougherty et al. indicate that improved energy storage can be achieved in a capacitor employing antiferroelectric dielectric layers by applying an electric field across the dielectric layers which enables a transition thereof through both an antiferroelectric phase and plural ferroelectric phases.
In both of the aforementioned Dougherty teachings, the antiferroelectric dielectric ceramics have comprised lead, lanthanum, zirconium and titanium (PLZT) constituents. The disclosure of Dougherty U.S. Pat. No. 5,545,184 and Dougherty et al. U.S. patent application Ser. No. 08/688,883 now U.S. Pat. No. 5,728,138 are incorporated herein by reference.
For capacitors which employ PLZT antiferroelectric ceramics, there is a need to achieve cost savings in their production by use of less expensive contact electrodes. For instance, certain PLZT antiferroelectric ceramics require application of sintering temperatures in the range of 1300-1350.degree. C. to achieve desired levels of densification. At such temperatures, platinum is the favored electrode material and adds substantially to the expense of the capacitor.
The prior art teaches that inorganic lithium compounds have been used as additives to ferroelectric ceramics to reduce the firing temperatures thereof. For instance, lithium salts have been used to lower the sintering temperature of various perovskite systems, especially BaTiO.sub.3, e.g., see Haussonne et al., "Barium Titanate Perovskite Sintered with Lithium Fluoride", Journal of the American Ceramic Society, Volume 66, No. 11, pp 801-807 (1983). Haussonne et al. found that when lithium fluoride and other lithium salts (lithium carbonate, lithium nitrate and lithium chloride) are added to barium titanate, the required sintering temperatures are lowered, as compared to barium titanate without lithium additions.
Laurent et al., "Sintering of Strontium Titanate in the Presence of Lithium Salts in a Reduced Atmosphere", Journal of Material Science, Vol. 23, pages 4481-4486 (1988) show that the addition of lithium salts reduce the sintering temperature of strontium titanate.
Fu and Chen, "Low Firing Dielectrics in the System Pb(Fe.sub.2/3 W.sub.1/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.0.9-x Ti.sub.0.1 O.sub.3 --Bi.sub.2 O.sub.3 /Li.sub.2 O", International Journal for Hybrid Microelectronics, Volume 10, No. 4, pages 1-5 (1987), studied the effect of bismuth oxide and lithium oxide on the sintering of the lead-based perovskite system: Pb(Fe.sub.2/3n W.sub.1/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.0.9-x Ti.sub.0.1 O.sub.3. It was observed that the addition of bismuth oxide/lithium oxide resulted in a sintering of the aforementioned ceramic at 850.degree. C. and a shifting of its Curie peak from 37.degree. C. to 10.degree. C.
Halmi et al. in "Improved Lead Perovskite Compounds (PFM-PFT) for Z5U Capacitor Applications" Advanced Ceramic Materials, Volume 3, No. 1, pages 32-37 (1988) observe that the addition of lithium enhances the sintering ability and dielectric permittivity of the ceramic. Each of the aforementioned teachings involved addition of a lithium salt to a ferroelectric ceramic, with the result being a lowering of the firing temperature.
Megherhi in a PhD thesis entitled "Interaction Studies of Lead Magnesium Niobate-based Capacitor Materials with Integrated Ceramic Packaging" (May 1991) found that the addition of lithium nitrate to lead magnesium niobate/lead titanate and lead magnesium niobate/lead zinc niobate relaxor perovskites enables a densification of the aforesaid ceramic materials at lowered sintering temperatures.
It is an object of this invention to provide an improved method for firing PLZT antiferroelectric ceramics.
It is another object of this invention to provide an addition to a PLZT ceramic which enables the firing thereof at a lower temperature than heretofore required.